The present invention relates to a method for making polyimides by effecting reaction between an aromatic bis-anhydride and an organic diamine under melt or solution polymerization conditions. More particularly, the present invention relates to the use of alkali metal salts of oxygenated phosphorus compounds as polymerization catalysts for making polyimides.
Prior to the present invention, certain catalysts, such as sodium chloride and ferric sulfate, as shown by Williams et al, U.S. Pat. No. 3,998,840, assigned to the same assignee as the present invention, were used to enhance the rate of formation of polyetherimide under melt polymerization conditions. Although valuable results were achieved by such procedures, it was found that the aforementioned catalysts were either less effective or not effective when utilized for making polyetherimides from aromatic bis(ether anhydride) and organic diamine under solution polymerization conditions. In addition, the presence of chloride ion may interfere with the use of such polyetherimide in electronic applications. Various metal compounds were also used to catalyze polyetherimide formation by imideamine exchange reactions, as shown by U.S. Pat. No. 3,847,870, Takekoshi and U.S. Pat. No. 350,885, Takekoshi et al, assigned to the same assignee as the present invention. However, the nature of the polymerization is quite different from polyetherimide formation by the melt polymerization of aromatic bis(ether anhydride) and organic diamine, which is amine-anhydride interaction rather than imide-amine exchange.